During the past few years, there have been frequent demonstrations of the fact that the appropriate selection of the most suitable crystalline modification significantly can influence the clinical results of a given chemical entity. The chemical and physical stability of a solid compound in a particular dosage form can be modified by presenting the substance in the appropriate crystal form. Little information is available on the role of polymorphism and crystal habit in solid dosage form and powder technology. It is, however, apparent that the appropriate selection of the most suitable crystalline modification, whether arising from polymorphic differences or as a result of solvate complex formation of beth water-soluble substances and less water-soluble substances, such as theophylline, often significantly can increase the medical value of a given drug in a particular dosage form. There are only a few statements available to predict the outcome of a crystallization procedure if e.g. the substance could be involved in different polymorphic or pseudopolymorphic forms. Solid-state transformations may also occur during mechanical treatment, e.g. micronization and by pressure during tableting. While a few generalizations can be made concerning the influence of structural modifications on the tendency of a chosen compound to exhibit polymorphism or other phenomena, a complete understanding of this problem awaits further research. Often "trial and error" approaches are used to develop a successful formulation of a drug. It is necessary to establish the conditions whereby different forms of a substance might be converted to a single form thus eliminating differences in solid-state properties and subsequent different physico-chemical properties.
E. Shefter and T. Higuchi have measured the relative rates of dissolution of several crystalline solvated and non-solvated forms of important pharmaceuticals, J. Pharm. Sci., 52 (8), (1963 ), 781-91.
L. van Campen, G. Zografi and J. T. Carstensen give in a review article an approach to the evaluation of hygroscopicity for pharmaceutical solids, Int. J. Pharmceut. 5, (1980), 1-18.
C. Ahlneck and G. Zografi describe the molecular basis of moisture on the physical and chemical stability of drugs in the solid state, Int. J. Pharmceut., 62, (1990), 87-95.
M. Otsuka et al. have calculated hydration data using various solid-state kinetic models for theophylline anhydrate powder, J. Pharm. Pharmacol., 42, (1990), 606-610.
Hak-Kim Chan and Igor Gonda have examined the properties of respirable crystals of cromoglycic acid by using different methods, J. Pharm. Sci., 78 (2), (1989), 176-80.
A more comprehensive discussion of factors relating to pharmaceutical preformulations and the physicochemical properties of drug substances is given by J. I. Wells in Pharmaceutical Preformulation: The Physicochemical Properties of Drug Substances, John Wiley & Sons, New York (1988). See particularly the chapter about polymorphism pp 86-91.